install and license office 2013 on terminal server

Hi

I have a customer who i have recently migrated to Office 2013 on all desktops and laptops.

They have a server 2008 terminal server running office 2003

I am trying to find out how i can install Office 2013 on the terminal server.

I believe that only Volume licensed copies of Office 2013 can be installed on a terminal server as these provide a .msi version of the software

Can i convert the laptops office 2013 licenses to volume licenses or do they have to be purchased separately?

Any advice on this matter is much appreciated

Regards

Colin